This invention relates primarily to an internal resistive system for television cathode ray tubes for protecting a tube and its associated circuitry from destructive electrical arcs and arc currents and for eliminating static charge accumulations inside the neck of a tube.
The envelope of a television cathode ray tube comprises a glass funnel and a mating faceplate. The funnel has a neck within which is located an electron gun. The faceplate of the tube has a fluorescent screen on which is impressed a very high DC voltage -- typically in the range of 20-30 kilovolts or more. The screen is stimulated by one or more electron beams generated in the gun.
Conductive coatings on the inside and outside of the funnel serve as a large capacitor which filters the high voltage supplied to the screen. The inner conductive coating is at screen potential and also serves to transmit the screen voltage to the neck of the tube where it is applied to a high voltage anode electrode at the forward end of the electron gun.
The electron gun has one or more cathodes and a series of closely spaced electrodes which shape, accelerate and focus the electron beam(s) generated in the gun. To accomplish these functions, the various electrodes require widely different electrical potentials. The large voltage differences established between certain high voltage and low voltage electrodes in the gun creates a susceptibility to arcing between the electrodes, e.g., shoudl there exist particulate foreign matter in an interelectrode space, a burr on an electrode, a misaligned or improperly spaced electrode, or the like. Large voltage differences between the gun electrodes and other tube internal components also establish arc-conducive conditions. When the conditions for arcing exist, the high voltage filter capacitor, with its immense stored electrical energy, will within a few microseconds or less dump its stored charge.
Because the instantaneous peak arc currents can reach hundreds of amperes in magnitude, great destruction can be wrought by such arcs. External circuitry can be damaged by transient currents and voltages induced in the associated receiver circuitry. Internal gun parts can be eroded to the point of inoperability or severely reduced in their effectiveness. High arc currents are capable of sputtering electrode materials onto adjacent surfaces, resulting in the formation of electrical leakage paths. Further, arcing in a tube during its normal operation can result in a loud audible report which may be quite disturbing to a viewer.
In recent years, the design evolution of color picture tubes has taken a direction tending to exacerbate the arcing problem. The desing for greater picture brightness has driven the screen voltages inexorably upward toward and even beyond 30 kilovolts. A trend toward wider beam deflection angles and a desire to minimize power consuption have dictated the use of tubes with smaller neck diameters. A small neck diameter implies a more closely confined environment for the electron gun, with the attendant increased probability of arcing between components of the electron gun assembly or between the gun assembly and the containing tube envelope.
In order to reduce tube arcing, it is routine today to design color tubes and electron gun assemblies with every effort to maximize intercomponent spacing, to minimize points of field concentration, and otherwise to configure the tube and gun structures to minimize the tendency of a tube to arc. After a tube receives its electron gun and the envelope is sealed, it is commonplace to "spot-knock" (high voltage condition) the gun. "Spot-knocking" is an operation wherein a pattern of fluctuating and constant voltages of high magnitude are applied to the tube to "knock" (remove) loose particles which may have lodged between gun electrodes, burrs on electrode parts, and other agents which might lead to arcing of the tube during its normal operation. Typically peak voltages during spot knocking are much higher than the screen operating voltage. Spark gaps, diodes, filters, gas discharge lamps, decoupling circuits, and other protective devices are commonly provided in the associated receiver (at significant cost) to protect receiver circuitry from damage by arc-induced currents and voltages.
Television picture tube manufacturers have long attempted to develop an internal resistive element which would be coupled in series with the high voltage filter capacitor and the electron gun to suppress the magnitude of arc currents and thereby overcome the potentially destructive effects of arcing in the tube during tube operation.
The requirements for such an internal resistance element are, however, extremely severe -- so severe that prior to this invention no commercially acceptable internal resistive element or system has been developed. Following are some of the requirements, not necessarily in their order of importance, of an internal resistive element for protecting television picture tube against arcing.
Requirement 1. The resistive element must be compatible with the clean high vacuum environment inside a cathode ray tube. The element cannot emit gas which might significantly decrease the tube's vacuum level or impair the performance of the cathode in the electron gun assembly. The element cannot flake, erode, ablate, or otherwise generate particles which might block openings in a color selection electrode or lodge in a gap between gun electrodes.
Requirement 2. The resistive element must be compatible with the tube's fabrication processes. Perhaps the most severe of the fabrication processes are the high temperature cycles which a color tube is subjected to when the faceplate is sealed to the funnel and during exhausting (and sealing) of the tube. Temperatures may reach 430.degree. C or higher during these high temperature operations.
Requirement 3. The resistive element cannot be physically obtrusive to the electron beams. As noted, there is a very limited amount of space available in the neck of a television cathode ray tube, particularly a tube of the small neck type, and particularly in the region near the front of the electron gun. Because of this space limitation, it has proven to be difficult to design a non-obtrusive discrete internal resistive element.
Requirement 4. The resistive element must be capable of being satisfactorily electrically terminated at each end. If the resistive element is a neck coating, it has been found that even modest arc currents are apt to cause localized heating of the glass underneath the contact point(s) with the result that the glass may chip or become predisposed toward eventual failure. It is difficult to maintain contact integrity with such an element after a number of arcs have occurred.
Requirement 5. The television industry being highly competitive, the resistive element and its cost of installation must be low enough to be commercially viable.
Requirement 6. Another requirement is that the resistive element not be susceptible to being by-passed by an arc as a result of the deposition of conductive material during flashing of a getter in the tube. Specifically, all television cathode ray tubes today utilize a "flashed" (vaporized) "getter" material which "gets" (adsorbs) residual gas in the tube after the tube has been pumped down as far as is practicable and sealed off. The gas-adsorptive getter material most commonly employed is a barium compound. Barium is highly conductive, however. When the getter is flashed, a conductive barium coating is deposited on substantially all exposed areas within the tube. In order to "get" the greatest quantity of residual gas, the getter must be flashed over a wide area inside the tube; inevitably, getter material is deposited on the resistive element. It is clear that any resistive element used for arc suppression or static elimination will be effectively by-passed or nullified if a shunt path around the resistive element or a major part thereof is created by conductive getter material.
Requirement 7. Yet another requirement is that the resistor not break down at operating or conditioning ("spot-knocking") voltages.
Requirement 8. A very important requirement is that the effective impedance of the resistive element be within an appropriate resistance range. If the dynamic impedance of the element is too low, e.g. below a few kilohms, inadequate suppression of arc currents will be provided. A resistive element may have an appropriate DC resistance measured outside of the tube but, when situated in a finished tube, be shunted by a stray capacitance which is so high as to establish a low dynamic parallel impedance across the element. It is believed that the afore-described stray capacitance problem has not been fully appreciated by prior practitioners in the art.
If the DC resistance of the element is too high (e.g. 10.sup.12 ohms), the material will act as an insulator and collect stray charges which may alter the electron beam paths or initiate arcing. Further, if the DC resistance of the element is too high, the voltage drop across the element as a result of gun leakage current flowing through it will result in an intolerable drop in the voltage applied to the anode electrode of the gun.